Kinematic, finite strain and vorticity analysis of the Sisters Shear Zone, Stewart Island, New Zealand
Introduction
Shear zones are arguably the most important deformation structures in the lithosphere. They are the most spectacular expression of the heterogeneity of deformation at all scales. In regions of lithospheric extension shear zones can accommodate tens or hundreds of kilometres of displacement (Foster and John, 1999, Wells et al., 2000, Ring et al., 2001, Thomson and Ring, 2006). Large-scale extension is commonly expressed by the development of metamorphic core complexes (Lister and Davis, 1989), which tectonically separate a mid/lower crustal core in the lower plate from non-metamorphosed sediments in the upper plate. The displacement is usually concentrated into a few tens to a few hundreds of meters thick ductile shear zone at the top of the exhuming metamorphic core. This ductile shear zone is overlain by a brittle detachment fault.
Ramsay and Graham (1970) advocated a model of progressive simple shear for ductile shear zones. Quantitative studies over the last two decades have shown that the deformation in shear zones is more complex and usually involves a coaxial component (Simpson and De Paor, 1993, Northrup, 1996, Grasemann et al., 1999, Ring, 1999, Kumerics et al., 2005, Bailey et al., 2007, Ring and Kumerics, 2008). In extensional terrains this pure-shear component is expressed by subvertical coaxial shortening leading to elongating shear zones (stretching faults in the sense of Means, 1989). Sullivan (2008) showed that strain intensity and strain geometry varies very significantly parallel to the tectonic transport direction in the Raft River low-angle extensional shear zone in the Basin and Range province. What is less well investigated are vertical and temporal variations in deformation (strain, rotation, translation) in shallow-dipping extensional shear zones.
The metamorphic core complexes on the west and south coast of the South Island of New Zealand are generally interpreted as precursors of late Cretaceous Tasman Sea and Southern Ocean rifting (Tulloch and Kimbrough, 1989, Gaina et al., 1998). The most prominent of the New Zealand core complexes is the Paparoa core complex (Fig. 1), which had a major phase of extensional faulting at ∼115–110 Ma when the upper amphibolite-facies, in part migmatitic, footwall cooled at rates of ∼100 °C Myr−1 (Spell et al., 2000, Schulte et al., 2014). The Paparoa core complex is a bivergent core complex characterized by two oppositely dipping and displacing detachments. Modelling work by Gessner et al. (2007) suggests that a bivergent symmetry develops when the lower crust is hot and has a low viscosity during extension and there is a pronounced strength difference between the upper and lower crust. A monovergent core complex, i.e. one that is characterized by a single shear zone/detachment system, develops when the viscosity contrasts between upper and lower crust are less, i.e. the mid/lower crust is stronger. These core complexes usually expose greenschist-facies rocks in their metamorphic core.
Another metamorphic core complex in New Zealand is the Pegasus core complex on Stewart Island (Fig. 2) (Kula et al., 2009). The overall geometry of the Pegasus core complex is not well known. Kula et al. (2009) reported along-strike differences in the kinematics of the Sister Shear Zone (SSZ), which is separating the Pegasus core complex from overlying sediments of the Great South Basin (Fig. 2). The southwest part of the SSZ is supposed to have a top-N displacement that is thought to be associated with a hypothetical top-S shear zone giving the structure an overall bivergent symmetry. The northeast part of the SSZ is supposed to be a monovergent top-S shear zone (Kula et al., 2009, their Figs. 2 and 11). In some respect this interpretation suggests that the Pegasus core complex has a mixed symmetry, i.e. bivergent symmetry in the SW and monovergent symmetry in the NE. Kula et al. (2009) did not report any quantitative structural data to support their along-strike differences in shear zone kinematics and overall geometry. Muscovite, biotite and feldspar 40Ar/39Ar by Kula et al. (2009) suggest that footwall cooling started at ∼94 Ma with accelerated cooling between 89 and 82 Ma.
We report the results of a kinematic, strain and rotation analyses from the SSZ for further constraining its kinematics and geometry. Our results suggest that top-to-the-SSE and top-to-the-NNW shear sense indicators resulted from a relatively high degree of coaxial deformation with the pure/simple shear ratio increasing downwards in the SSZ. Our data do not support a model in which the kinematics of the shear zone changed along strike.
Section snippets
Setting
The Mesozoic geology of New Zealand was dominated by a long-lived subduction zone at the eastern margin of Gondwana (Fig. 1). In this subduction complex an Eastern and a Western province are distinguished. The forearc terranes of the Eastern Province are mostly trench-fill sediments. The Western Province is made up by the subduction-related magmatic arc (Median Batholith) and its surrounding rocks (Mortimer, 2004).
At about 110 Ma the subduction system was shut off and the New Zealand continent
Finite strain analysis
To describe finite strain in a rock six independent variables are required. Three variables describe the orientation of the principal stretching directions X, Y and Z, where X ≥ Y ≥ Z. The remaining three variables describe the magnitude of the principal strain along these directions, which are represented by the absolute principal stretches SX, SY and SZ (SX ≥ SY ≥ SZ). The stretches are defined by lf/li, where li and lf are the initial and final lengths of a material line. For practical
Structures in the SSZ
We describe the structures in the SSZ from bottom to top. The deeper, higher-grade deformation fabrics are exposed in Pegasus Bay, especially well around South Arm, whereas higher parts of the shear zone with lower greenschist-facies structures are preferentially exposed along the coast of SE Stewart Island (Fig. 2).
Overall, the various granites in the SSZ show the typical heterogeneous deformation of metagranites with tens of meters thick mylonitic to ultramylonitic zones alternating with much
Finite strain
The finite-strain data are shown in Fig. 6. Most strain ellipsoids have an oblate symmetry (14 samples), but 2 have prolate symmetries and one sample plots near the plane-strain line. The scatter in strain symmetry indicates that deformation in the shear zone was heterogeneous. The most extremely deformed samples are characterized by upper/mid greenschist-facies deformation and usually have strong oblate symmetry with pronounced extension in the Y direction. Those samples are from the deep
Apatite fission-track ages
The AFT ages are shown in Table 1. Most samples pass the chi-square test, but track counts are overall low, and as a consequence chi-square values may be upward biased. For eight samples we could measure tracks lengths, but the number of available confined horizontal tracks was rather limited (2–44 per sample). For those grains where tracks-lengths were measured Dpar values were also determined, which range between 1.74 and 2.36 μm (Table 2).
Comparison with other age data
Kula et al., 2007, Kula et al., 2009 reported 40Ar/39
Deformation regime in the SSZ
The maximum stretching direction during movement in the SSZ was NNW-SSE oriented and is reflected by the stretching lineations measured in the field (Kula et al., 2009; this study). Our data show that deformation deviated significantly from progressive simple shear and involved a strong component of vertical flattening perpendicular to the shear zone walls. Associated with the general shear deformation are conflicting kinematic data showing alternating top-to-the-SSE and top-to-the-NNW senses
Conclusions
We have shown that the SSZ is a general shear zone whose kinematic development involved a significant component of pure shear flattening. The data show that only the upper SSZ near the Sisters Detachment had a significant simple shear component suggesting that the rotational component of shearing was partitioned into the detachment interface between foot- and hangingwall. Our quantitative data also indicate along-strike kinematic continuity along the SSZ. The AFT ages and T-t models and the
Acknowledgements
We thank Virgina Toy and an anonymous reviewer for commenting on the manuscript and Joao Hippertt for editorial handling. Tulloch acknowledges research funding from the New Zealand Government.
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